This invention relates generally to the field of telephony, and more particularly to an improved protector module of a type used to protect individual subscriber circuits from excess voltage and current surges. More particularly, it relates to a current surge protective means in modules of this type offering improved protection necessary to protect modern solid state circuitry now used by telephone companies in newer installations.
With the advent of recently published narrow window voltage requirements of the ESS No. 5 specifications, there has arisen a need for a module which will fire within a range of 215 to 265 volts, and do so as quickly as possible in order to avoid damage not only to the protective circuitry, but the module itself. Within this voltage range, conventional heat coils may require as much as 30 to 40 seconds to melt the fusible component of the heat coil assembly permitting the establishment of a direct path to a source of ground potential bypassing the gas tube elements.
In traditional heat coil construction, it is common to provide a cylindrical solder pellet which melts at relatively low temperatures. Other constructions employ a thin film of fusible material between a heat coil bobbin and a member slidably disposed within the bobbin. In the former case, a considerable amount of heat is necessary to melt to solder pellet. In the latter case, less heat is necessary, but the bobbin itself and the sliding member disposed therein must also be heated before the fusible interconnection melts.
Given the normal resistance of individual subscriber circuits, during the course of time required to melt conventional fusible components, the gas tube elements are required to transmit amounts of current for exceeding their normal capacity. This excess current manifests itself as heat, which if unabsorbed, can destroy the housing which encases the module, or so badly distort it that removal from a protector block is difficult if not impossible. It is to be appreciated that each protector module is normally installed in very close proximity to similar protector modules servicing other subscriber circuits, and that damage to the housing element of one such module through internally generated heat can often at least partially damage those other modules adjacent its periphery.